1. Field of the Invention
The present invention relates to a method for removing luminance nonuniformity and crosstalk occurences on a display using organic electroluminescence and an organic electroluminescence display manufactured by this method.
2. Description of Related Art
Organic electroluminescent elements are configured by sandwiching a material layer between an anode and a cathode. The material layer may comprise a plurality of layers, such as an electron-injecting layer or a hole-injecting layer and an electron-transporting layer or a hole-transporting layer. Its emitting principle is similar to that of the emitting mechanism of light emitting diodes (LED). More specifically, a hole and an electron are fed into a light-emitting layer by the application of a direct current voltage between the anode and the cathode. The electronic state of organic molecules included in the light-emitting layer is changed to the excited state by energy generated by a recombination of the hole and electron in the light-emitting layer. Energy is emitted as light when this quite unstable electronic state falls to a ground state. Accordingly, organic electroluminescence is referred to also as organic light emitting device (OLED).
In an OLED display, OLED elements are arranged on a substrate, such as a glass substrate, as a matrix to emit light to show information. OLED displays are expected to gain a substantial market share because of their superiority in electric power consumption, reaction speed, visual field, and luminance compared with other types of displays, such as liquid crystal displays.
A method for driving OLED elements is roughly divided into two kinds of systems: a passive matrix system and an active matrix system. As shown in FIGS. 5(a) and 5(b), the passive matrix system is a driving method to intersect an anode 114 and a cathode 116 in a matrix state to selectively activate OLED elements sandwiched at an intersection. On the other hand, as shown in FIGS. 6(a) and 6(b), the active matrix system is a driving method to activate OLED elements by having switching and memory functions for each pixel 130 using a thin film transistor (TFT) 120.
Using the passive matrix system enables low production costs of displays because of its simple structure. However, large electric power consumption is required to keep the screen at high luminance because this system indicates information by sequentially emitting lines and using an after-image retained by the eyes. For this reason, the active matrix system for activating the pixels 130 with energy from the TET's 120 has been adopted more frequently despite its high production costs. Compared with the passive matrix system, the active matrix system produces a high luminance at a low electric power consumption.
An OLED display 110 has two systems for emitting luminance: bottom-emitting system and top-emitting system. As shown in FIG. 7(a), the bottom-emitting system takes out light from an insulating substrate side 118. As shown in FIG. 7(b), the top-emitting system takes out light from a top surface layer 115.
Japanese Patent Publication No. 8-227276 discloses embodiments of a method of manufacturing bottom-emitting and top-emitting OLED displays. According to these embodiments, an OLED display shown in FIG. 10(a) is manufactured by the processes shown in FIGS. 10(b) to 10(d). More particularly, as shown in FIG. 10(b), a plurality of parallel first display electrode lines 214 made of indium tin oxide (ITO) or the like are deposited as stripes on a glass substrate 218. Ribs or walls 222 of polyimides or the like are formed on the first display electrode lines 214, so that island-shaped first display electrode portions 215 are defined and surrounded as shown in FIG. 10(c). An OLED light-emitting layer 213 is formed on each recess of the glass substrate 218 wherein the ribs (walls) 222 are formed. Next, a plurality of parallel stripe second display electrode lines 217 of low resistance metal are vacuum-deposited or sputtered with a shadow mask with parallel slits on the ribs 222 and the light-emitting layers 213 so that the second display electrode lines 217 extend perpendicular to the first display electrode lines 214.
In the area surrounded by the ribs 222, a TFT connected to the first display electrode portions 215 is formed on the glass substrate 218, where data signal lines and scan signal lines or the like are arranged. As shown in FIG. 10(a), in this embodiment, the OLED display emits light from the glass substrate side 218.
In the active matrix system, the aperture ratio is reduced due to TFT, capacitors, and wiring or the like when passing light through the glass substrate side 218 in the bottom-emitting system. Consequently, when the active matrix system is adopted, the top-emitting system is advantageous. Light is not shielded by the TFT, which results in an increase of the aperture ratio and high luminance when adopting the top-emitting system.
FIG. 11 shows a cross sectional view of the structure of a top-emitting active matrix OLED display. In FIG. 11, an OLED display 310 comprises: an insulating substrate 318; a thin film transistor (TFT) 320 formed on the insulating substrate 318; an insulating layer 319; a first electrode 314; a material layer 313; a second electrode 317; and a virtual hole 326 for connecting the first electrode 314 and the TFT 320 through the insulating layer 319.
Unlike the bottom-emitting system, the second electrode 317 is required to be made from a transparent material because the OLED display 310 emits luminance through the second electrode side 317. Further, to increase optical transmittance, the second electrode 317 needs to be as thin as possible. Moreover, the second electrode 317 may be laminated covering the entire surface of the OLED display.
A light-emitting layer included in the material layer 313 of the OLED display 310 emits light which passes through the second electrode side 317.
Since the structure of such top-emitting active matrix OLED displays is various, the second electrode 317 covering the entire surface of the above-mentioned OLED display may be divided into stripes as in the case of the passive matrix system. Further, the virtual hole 326 in the layer 319 connects the first electrode 314 to the TFT 320, and may be used to connect, for example, the second electrode and the common electrodes.
One example of a top-emitting active matrix OLED display having ribs will be now described with reference to FIGS. 8(a) and 8(b).
AS shown in FIG. 8(b), in an OLED display 110, ribs 122 are arranged on an insulating substrate 118 in parallel. As shown in FIG. 8(a), OLED elements 112 are sandwiched between ribs 122. The area of one unit of matrix divided by the ribs 122 and OLED elements 112 are referred to as a cell area 132. The cells, completed by equipping the cell area 132 with the TFT 120 and the OLED elements 112, are referred to as pixels 130.
The pixels 130 in each cell area 132 are so configured that an anode 114 and the ribs 122 are formed on the insulating substrate 118 by sandwiching the anode 114 in the column direction of the matrix in parallel as shown in FIG. 8(a). Further, in parallel with the ribs 122, common electrodes 124 isolated from the anode 114 and the ribs 122 are formed on the insulating substrate 118. Furthermore, the OLED elements 112 are formed by the lamination of at least a light-emitting layer and a thin film cathode 117 on the upper part of the anode 114. Moreover, the thin film cathode 117 is laminated on the pixels 130. And the virtual holes 126 for conducting the thin film cathode 117 and the common electrodes 124 may be formed in each cell area 132.
The thin film cathode 117 is laminated on the entire surface of the OLED display 110. The thin film cathode 117 is partitioned by the ribs 122 formed among the adjacent cell areas 132 in a column direction when laminating the thin film cathode. The anode 114 is not needed to be optically transparent in top emitting system but may be made from a metal, such as Al.
Additionally, the cell areas 132 are rectangular in shape. Each cell area 132 includes OLED elements 112. The common electrodes 124 are formed on the insulating substrate 118 in parallel with the ribs 122 to be isolated from the anode 114. The common electrodes 124 may conduct with the thin film cathode 117 through the virtual holes 126 formed within each cell area. Accordingly, the thin film cathode 117 laminated on the surface of the OLED display 110 is equipotential through the common electrodes 124.
When an OLED display 110 having such configuration is driven employing the active matrix and top-emitting systems, a circuit formed by circuit elements, such as the OLED elements 112 and common electrodes 124 as are shown in the schematic diagram 4(a) or 4(b) as an ideal example. More specifically, the OLED elements 112 emit light by the application of a forward voltage between the OLED elements 112 through the TFT because of this mechanism. For example, in FIG. 4(a), a current passing through the OLED elements passes into the common electrodes 124 from the surface of the thin film cathode 117. The following explanation is given using the schematic diagram 4(a) for convenience sake.
Considering a circuit as shown in FIG. 4(a), a predetermined amount of current always passes through the OLED elements 112 which are selected by applying a certain voltage. A current does not always pass through the OLED elements 112 which are not selected. On the other hand, it is known that the luminance of the OLED elements 112 is approximately proportional to the current passing through these OLED elements 112. It follows that the light emission of the selected OLED elements 112 is performed at predetermined luminance while the light emission of the unselected OLED elements 112 is never performed, which results in no unexpected luminance nonuniformity.
Upon driving the OLED display 110 having the above-mentioned configuration, however, as shown in FIG. 9, it has turned out that an apparent linear luminance nonuniformity appears on the surface of the display. Especially, such linear luminance nonuniformity distinctly appears on top-emitting active matrix OLED displays wherein ribs 122 are arranged in parallel, as in the above-mentioned systems. Further, luminance nonuniformity in a spot shape easily occurs on the kind of OLED displays without ribs 122, wherein the entire surface is covered with a thin film electrode.